This invention relates to a novel method of measuring shot peening coverage by using correlation analysis of surface line data. More particularly, said method comprises the use of cross correlation of two or more parallel profile traces of a shot peened surface to produce a numeric indication of relative part coverage with a minimum of computational delay.
During a shot peening process, a stream of shot traveling at a high velocity is directed at a workpiece surface. The shot is directed at the workpiece so as to cause plastic deformation of the workpiece surface, which often is a metal surface. Although the process may be applied for other purposes, the shot peening process generally is used to increase fatigue strength of the workpiece.
In a shot peening process, the ability to attain accurate coverage levels is important to the quality of the finished product. This is especially true for contoured or repeened surfaces where it is difficult to predict the exact coverage or know the coverage history.
Under normal shot peening conditions, it is desirable to expose the substrate or work surface to the shot stream for a sufficient time to achieve 100% surface coverage, or sufficient exposure to fully cover the surface with impact dimples. Insufficient coverage has obvious adverse consequences. If the local compressive zones, due to individual shot strikes, do not merge into a continuum that fully covers the surface with a compressive layer, locally unprotected regions will exist which may cause some regions of the surface to remain in tension and become crack initiation sights. In addition, too much coverage will overharden the surface, making it become brittle and susceptible to cracking.
To date various coverage analysis techniques exist. Several of these techniques are disclosed in U.S. Pat. No. 5,235,517 and U.S. Pat. No. 5,293,320, both being invented by R. A. Thompson and Mark A. Tascillo, inventors of the instant invention. Each of the before mentioned patents have been assigned to the same assignee as the present invention.
The techniques disclosed in U.S. Pat. No. 5,235,517, "Measurement of Shot Peening Coverage by Automated Analysis of Peened Surface Line Traces," rely on the detection of increasing surface hardness as surface exposure time to the shot stream increases. This approach works well with multiple single data traces and shows good response where periodic machining information has been removed from the part by the peening process. The hardness algorithm is efficient but suffers from a somewhat diminished signal to noise ratio due in part to taking derivatives of the data.
The approach taken in U.S. Pat. No. 5,293,320, "Measurement of Shot Peening Coverage by Impact Dent Characterization," computes a contour map of the surface from a high resolution 3-D surface scan. The contour map is then processed to resolve the individual shot strikes and totals them. This approach maintains good resolution of coverage up to 400%, after which the additional peening effects are no longer resolved. This analysis has proved the most accurate to date but is numerically intensive and requires considerable processing time. Furthermore, the on-line use of this algorithm requires high resolution, multi-trace 3-D surface scans. Such a process is practical for smaller items but becomes more difficult for larger, more ungainly objects.
An ideal coverage analysis for use on a factory floor should work from line data and require the accumulation of only a few line traces. In addition, the computation time should be minimal.
It is apparent from the above that there exists a need in the art for a quick, inexpensive, and accurate method of measuring shot peening coverage. It is a purpose of this invention to fulfill this and other needs in the art in a manner more apparent to the skilled artisan once given the following disclosure.